Have you ever considered that perhaps when one of thhe classes die they come bak not whole but slightly robotic, but not entirely human. then the robots could still tehinically inflict damge upon them. So if you look atit this way before mann vs machine each of the mercs must have died at least ounce in battles with red and blu mann making them undead, ihuman and imune to roboethics because robots can still techinally "kill" other robots.